There are two forms of angular field according to the type of receiver associated with the telescope. For the linear receivers, the angular field value is a few degrees in a first direction of space and a few tenths of degrees in the perpendicular direction. For the matrix receivers, the angular field value is a few degrees in both directions of space.
The optical architecture of this type of telescope comprises only conventionally off-axis mirrors. This type of architecture makes it possible to produce compact telescopes, having very good transmission and totally without chromatic aberrations. The image quality also has to be excellent in all the field. Consequently, the optical architecture has to be perfectly corrected of the geometrical aberrations that are spherical aberration, coma, field curvature and astigmatism.
A number of optical solutions have been proposed to produce such architectures.
A first type of optical architecture for anastigmat telescopes comprises three mirrors. These telescopes are also called “TMA telescopes”, from the terminology “Three-Mirror Anastigmat”. Conventionally, the mirrors of a TMA telescope are not inclined or “tilted”. If the mirrors are all on a common optical axis, there is a significant central occulting. To eliminate the central occulting, either an “off-axis” field and/or “an off-axis” pupil is produced. In effect, the mirrors can be tilted to eliminate the central occulting, but this solution adds geometrical astigmatism and eccentricity coma aberrations which are not generally acceptable.
There are TMA telescopes in which the mirrors are slightly tilted and/or off-centre. Generally, the tilting of the mirror does not exceed one or two degrees. This optical solution makes it possible to reduce the field off-axis margin and/or pupil margin, but not totally eliminate them. An example of this type of three-mirror telescope is represented in FIG. 1. In this figure and the subsequent figures, the following conventions have been adopted. The figures are views in a cross-sectional plane. The mirrors are represented by bold lined circular arcs. The photosensitive detector D of the telescope is represented by a rectangle. Also represented are two light rays representative of the pupil edge rays for the central field. These light rays are represented by thin lines. In FIGS. 2 and 7, the intermediate planes are represented by dotted lines.
In the case of FIG. 1, the three mirrors are aspherical. The first mirror M1 is concave, the second mirror M2 is convex and the third mirror M3 is concave.
The TMA telescopes offer significant linear fields. Thus, the linear field can exceed 15 degrees. However, with given focal length, their bulk is sizeable and becomes prohibitive for certain applications, particularly when the pupil of the telescope has a significant diameter or when the focal length is significant.
There is also a second type of optical architecture that is more compact than the preceding architecture. These telescopes are called “Korsch”. Their architecture represented in FIG. 2 is a variant of the preceding architecture. The Korsch telescopes are also a system with three aspherical mirrors M1, M2 and M3 of concave-convex-concave type, but the optical system has an intermediate focal plane PFl between the second mirror M2 and the third mirror M3. The mirror MR of FIG. 2 is a simple planar return mirror and is not involved in the optical system. Unfortunately, their field is limited. Thus, the linear field cannot readily exceed three degrees.
By way of example, a Korsch telescope with 10-metre focal length with f/4 aperture can have a linear field of 3°×0.5°. In this case, the root mean square error on the wavefront, or RMS WFE, the acronym for “ Root Mean Square WaveFront Error” does not exceed λ/20 throughout the field of the telescope.
As has been stated, the mirrors used in the optical systems of the TMA or Korsch telescopes are aspherical mirrors. More specifically, their surface is defined by a conic and aspherical terms of revolution. Now, these surfaces are not perfectly adapted to correct the aberrations of optical systems which no longer have an axis of symmetry like the TMA or Korsch telescopes. The conventional TMAs have symmetry of revolution. However, they are not used on their optical axis, but in the field. The TMAs are perfectly corrected of the aberrations at the centre of the field, on the optical axis, but the occulting renders this point of the field inaccessible. The field of the telescope is therefore off-centre.
The more the distance from the optical centre increases, the more the image quality decreases because the system is not perfectly corrected of the aberrations. Thus, the RMS WFE of the preceding Korsch telescope changes to λ/4 when the linear field changes from 3°×0.5° to 6°×0.5°. This error is no longer compatible with the performance levels required. This is a first limitation.
Moreover, in a conventional Korsch, a hole has to be made in the first mirror in order to allow the light to pass as can be seen in FIG. 2. The first drawback associated with the presence of this opening is a reduction of the useful surface area of the primary mirror of the order of 15 to 20%. The second drawback is of a mechanical nature. For a field less than 3°, the hole dimension is acceptable, but if the field of view is increased, the size of the hole becomes significant and makes it necessary to produce the mirror M1 in two distinct parts, which poses significant mechanical problems. This point is illustrated in FIG. 3. On the left in this figure, a mirror M1 is represented with an aperture TM1 that is sufficient to allow a field of 3 degrees by 0.5 degrees to pass. On the right in FIG. 3, a mirror is represented in two parts M1′ and M1″ separated by the aperture TM1, the two parts being necessary to allow a field of 6 degrees by 0.5 degrees to pass. Finally, the aperture of the mirror necessarily degrades the modulation transfer function or FTM of the telescope as can be seen in FIG. 4 where there are represented on the one hand the real FTM of the telescope with its aperture and the theoretical FTM without aperture.
In recent years, a new type of optical surface has been developed. These surfaces are known as “freeform” surfaces. Generally, a freeform optic is a surface which has no symmetry of revolution.
There are various definitions of the freeform surfaces. Generally, each definition addresses a particular need, is adapted to a specific computation and optimization mode and, of course, to a specific embodiment.
By way of examples, the mathematical definitions of a freeform surface can be as follows:
Freeform surface defined by polynomials XY. In clear, this surface being defined in a space (x, y, z), if z(x, y) represents the coordinate z of a point of this surface, the following relationship applies:
      z    ⁡          (              x        ,        y            )        =                    c        ⁡                  (                                    x              2                        +                          y              2                                )                            1        +                                            1              -                              (                                  1                  +                  k                                )                                              ⁢                                    c              2                        ⁡                          (                                                x                  2                                +                                  y                  2                                            )                                            +          ∑                        A          i                ⁢                  x          j                ⁢                  y          k                                    C being the curvature of the surface, k being the conicity constant, Ai being constants, i, j and k being indices varying respectively between 0 and three integer numbers.        
This surface corresponds to an extension of the conventional definition of the aspherical surfaces by generalizing it to a surface without symmetry of revolution;
Freeform surface defined by phi-polynomials, for example the Zernike or Q-Forbes polynomials. The Zernike surfaces are the most commonly used. A Zernike surface is defined in polar coordinates in a space (ρ, φ, z), if z(ρ, φ) represents the coordinate z of a point of this surface, the following relationship applies:
      z    ⁡          (              ρ        ,        φ            )        =                    c        ⁡                  (                      ρ            2                    )                            1        +                                            1              -                              (                                  1                  +                  k                                )                                              ⁢                      c            2                    ⁢                      ρ            2                                +          ∑                        C          j                ⁢                  Z          j                                    Zj being a j order Zernike polynomial and Cj being the constant associated with this polynomial, j being an index varying respectively between 0 and an integer number.        
The publication by G.W. Forbes entitled “Characterizing the shape of freeform optics” 30.01.2012/Vol.20, N° 3/Optics Express 2483 describes the surfaces defined by the Q-Forbes phi-polynomial surfaces.
Freeform surface defined by local equations of freeform surfaces of different definition.
Freeform surface defined by hybrid descriptions such as, for example, surfaces mixing phi-polynomial surfaces and so-called “NURBS” (Non-Uniform Rational Basis Splines) surfaces.
These freeform surfaces have been used to produce three-mirror telescopes. A first architecture of this type is represented in FIG. 5. The architecture is a triangular system with three mirrors, convex—concave—concave, of which at least two of the three mirrors are freeform surface mirrors. A description of this can be found in a number of publications including the patent U.S. Pat. No. 8,616,712 entitled “Nonsymmetric optical system and design method for nonsymmetric optical system”. This optical solution makes it possible to achieve significant fields but does not have the requisite compactness.
A second architecture of this type is represented in FIG. 6. The architecture is also a system with three mirrors, convex—concave—concave, of which at least one of the three mirrors is a freeform mirror. A description of this can be found in a number of publications including the patent application US 2014/0124649 entitled “Off-axial three-mirror system”. This optical solution makes it possible to achieve significant fields but, hereagain, does not have the requisite compactness when the focal length is of significant size.